Lasers are commonly used today in a variety of medical applications. Lasers have been used dermatologically to destroy skin tumors and remove unwanted skin pigmentation. Physicians have utilized lasers to destroy lithic concretions within bodily cavities, such as kidney stones. Lasers have been used extensively to cut and ablate human tissue, particularly at wavelengths and intensities that result in the coagulation of blood and cauterizing of small vessels contemporaneous with incision making. Called "bloodless" surgery, photocoagulation techniques have found great utility in treating eye disorders, particularly those associated with the retina. Lasers are particularly useful in this regard because of the difficulties in access which internal eye surgery presents and because of the need to prevent blood from entering the vitreous humor of the eye.
Notwithstanding the great advances made in eye surgery using lasers, many problems still remain. One of these is damage to the cornea resulting from laser passage through the cornea on its way to the retina. Although appearing transparent, the cornea and the fluids present between the cornea and the lens do absorb laser energy. In the case of the elderly, cataracts are common resulting in much greater energy absorption than through a normal cornea. Likewise, the subcorneal humors can trigger an immune response which results in a clouding of the fluids and consequent increase in light absorptance.
In conducting any sort of transcorneal laser therapy, energy density of the laser beam as it intersects the cornea must be limited. High energy absorptance by the cornea can result in damage. This problem is compounded by the fact that physicians must use special contact lenses on their patients in order to be able to visualize the placement of the laser beam on the patient's retina. Many of these lens actually increase energy density to the cornea because they have the effect of reducing the diameter of the portion of the laser beam near the cornea compared with what would otherwise be the case. Thus, physicians have sought to minimize the damage potential to the cornea while at the same time maintaining adequate energy densities at the retina necessary for therapy. In many instances, because a focused or parfocal beam is preferred, physicians will chose a lower energy setting and smaller spot size.
An alternative way in which corneal energy densities have been addressed is through the use of a defocused beam, that is, a beam that intersects the retina at a point spaced from the focal plane (i.e., the narrowest portion) of the laser beam. Delivery of a defocused beam of laser radiation to the retina can result in much lower energy densities in the cornea than would be possible if the beam were delivered in a focused mode.
Aside from prevention of damage to the cornea, some physicians find a defocused beam to be more appropriate in certain therapies, particularly where a sharply defined treatment area is not desirable. In these instances, physicians prefer to use a defocused beam.
Generally speaking, known systems are designed to deliver a beam only in a focused or a defocused mode. Some laser systems exist, such as Coherent Model No. 920, which are able to deliver a focused beam for one range of spot sizes and a defocused beam for another range of spot sizes. In the latter range, the spot size is varied by moving the focal plane of the laser beam out of the plane of the retina. No existing devices, however, are able to provide the physician with the option of choosing a focused or a defocused spot for an overlapping range of spot sizes.
Notwithstanding the difficulties associated with corneal damage and the need to use both a focused and defocused beam, prior art laser systems are designed to deliver laser beams in only one mode, focused or unfocused, for a given set of laser beam spot sizes. Thus, there exists a need for a single laser system for treatment of the eye that is capable of delivering a laser beam alternatively in a focused or defocused mode as desired for a range of beam spot sizes.